1 Field of the Invention
The present invention relates to aqueous dispersions of poly(meth)acrylates for forming finishing coats on solid plastic surfaces by means of varnishing, a method for varnishing plastic articles with such aqueous dispersions, and plastic articles which have been varnished with such aqueous dispersions.
2.Discussion of the Background
After polymerization of plastic raw materials into semi-finished or finished products, the formed parts thus-obtained are usually given an additional surface treatment. This surface treatment, which at its simplest may take the form of coating with a varnish, may be done both for both decorative and functional reasons. In practice, these two reasons for varnishing plastic objects overlap in most cases. With a clear or pigmented varnish, for example, one can improve the luster or impart a variety of colors to the plastic surfaces, while at the same time substantially increasing the weather-resistance of plastics, such as polycarbonates, acrylonitrile-butadiene-styrene polymers and polyphenylene oxide, which are subject to severe photochemical degradation when exposed to weathering.
The varnish system used for varnishing the shaped parts is selected according to the nature of the surfaces and the particular requirements for the varnish finish coat. Generally used for this purpose are oxidatively drying fatty acid containing synthetic resin varnishes, chemically vulcanizing multiple-component varnishes of epoxide, polyurethane or unsaturated polyester resins and primarily hard polyacrylates, i.e., varnishes with a relatively high proportion of poly(methyl methacrylate). The polyacrylate systems have the advantage over the other varnish systems in their weather-resistance, i.e., they do not yellow and are thus not subject to degradation, and therefore the mechanical properties of the coatings are stable, they are ethanol- and water-resistant and are simple to apply to surfaces in their organic solutions. (Ullmann, Encyklopadie der Technischen Chemie, 4th Ed., Vol. 15, pp. 343-345).
For reasons of both environmental protection and work safety, and also because of the danger of acceleration of the formation of stress cracks and the consequent change in the mechanical properties of the plastics due to the effects of organic solvents, it would be an advance to replace solvent-containing varnishes with aqueous systems.
DE-A 30 07 936 discloses aqueous polymer dispersions for the formation of protective and decorative coatings, on plastic substrates among others, which are combination dispersions composed of cellulose esters and poly(meth)acrylate and/or copolymers of poly(meth)acrylates, and which pursuant to the disclosure contain fatty acid modified resins such as oil or fatty acid modified alkyd resins, and which offer the important advantages over prior art cellulose ester/poly(meth)acrylate combination dispersions of surface hardness and good alcohol resistance when used as coating materials. Aqueous dispersions that are prepared by polymerization of (meth)acrylic acid esters containing dissolved cellulose esters and which according to DE-A 30 07 936 yield coatings with inadequate properties are described in EP-B 0 010 424 (corresponds to U.S. Pat. No. 4,252,697) and in DE-A 32 00 765 (corresponds to U.S. Pat. Nos. 4,415,703 and 4,521,565). Suitable monomers for the preparation of the cellulose ester containing poly(meth)acrylate phases according to the above-mentioned patents are acrylic and/or methacrylic acid esters with alkyl, cycloalkyl, phenyl or benzyl radicals as ester radicals.
The preparation of varnishes pursuant to DE-A 30 07 936, however, proves to be disadvantageous and risky in many respects, since what is used for the cellulose ester, as is demonstrated throughout the examples, is nitrocellulose, which is dissolved in the monomers and other organic solvents and subjected to rather high temperatures for a rather long period of time during the polymerization of the monomers. The advantage of using aqueous systems instead of polymer solutions in organic solvents for producing the coating is largely lost in this process, because of the concomitant use of relatively large quantities of organic solvent in preparing the dispersion and the need for further large quantities of solvent for the preparation of the varnish from the polymer emulsions, as the description of their preparation shows.
Dispersions of (meth)acrylate polymers that are essentially obtained by emulsion polymerization of methacrylic acid esters and acrylic acid esters of the type in which the ester group contains straight-chain or branched alkyl groups with 1 to 8, preferably 1 to 4 carbon atoms, particularly the methyl and ethyl esters, constitute the common, commercially available poly(meth)acrylate dispersions. Attempts to produce plastic varnishes with given properties with dispersions of this kind surprisingly showed that the resulting coatings, in comparison with those prepared from solutions, in spite of having an identical or similar structure of corresponding monomers, did not display the requisite profile of properties. Dispersion polymers with relatively high minimum film formation temperatures (MFT &gt;80.degree. C.) produced virtually no films. Soft polymers used, i.e., polymers prepared with ester groups from long alkyl radicals, e.g., C.sub.4 to C.sub.8 alkyl radicals, yielded films that were adherent but sticky and not alcohol-resistant.
High resistance to alcohol is required, for example, for the plastic housings of many hi-tech consumer products, such as television sets, video equipment, phonographic equipment, and kitchen appliances, as well as computer housings.